1. Field of the Invention
The present invention relates to a projector which includes space modulation elements such as liquid crystal display panels for producing a modulated light and a projection lens for projecting the modulated light onto a screen for producing an image thereon. In particular, the present invention relates to a projector in which the modulated light projected by the projection lens is projected to the screen after the light is reflected by a back surface reflection mirror including a transparent plastic sheet and a reflecting film.
2. Description of the Related Art
Projectors can be classified into front surface type projectors and rear surface type projectors. In the front surface type projector, a reflective type screen is located on a vertical wall in a room, and a projection unit including liquid crystal display panels and a projection lens is arranged at the center in the room, whereby the modulated light is projected by the projection lens onto the screen for producing an image thereon. Observers can see the light reflected by the screen as an image. On contrast, in the rear surface type projector, a projection unit including liquid crystal display panels and a projection lens is arranged in the projector casing, and a light transmitting type screen is arranged on the front of the projector casing. Observers can see the modulated light which has passed through the screen.
Rear surface type projector having a larger screen, such as a screen with diagonal dimension in the range from 70 to 100 inches, has recently been developed. In the larger rear surface type projector, it is necessary that the distance between the projection lens and the screen is extended to, for example, 2.5 to 5 meters, and the projector casing must be enlarged. Accordingly, it has been proposed to arrange a mirror between the projection lens and the screen to reduce the depth of the projector casing.
Conventionally, a front surface reflection mirror made from glass is arranged between the projection lens and the screen. If a mirror is to be used for the above described purpose in a rear surface type projector having a screen having a diagonal dimension in the range from 70 to 100 inches, the size of the mirror will be, for example, 1.5 m.times.1.1 m. If such a large mirror is made from glass, the thickness of glass should be greater than 5 millimeters since glass is brittle and fragile. The weight of the mirror, which will be more than 20 kilogram, becomes a problem. In addition, the mirror is mounted to the projector frame by mirror holders, but the mirror holders must be also strong and heavy. Accordingly, the weight of the mirror assembly may be 20 kilogram to 50 kilogram, and the weight of the entire projector may be more than 100 kilogram.
Accordingly, it is conceived to make the mirror arranged between the projection lens and the screen from a plastic sheet which can be made thin and light because plastic is elastic and not brittle and because the specific weight of plastic is approximately 60 percent of the specific weight of glass.
The inventors have attempted to make a mirror from a transparent plastic sheet. The mirror was a back surface reflection mirror including a transparent plastic sheet and a reflecting film superposed on the plastic sheet. The light projected by the projection lens was made incident to the plastic sheet, was reflected by the reflecting film located on the back surface of the plastic sheet, and emerged from the plastic sheet. It was possible to reduce the thickness of the mirror to 100 .mu.m or less by which the mirror is not damaged, and accordingly the weight of the mirror can be considerably reduced.
However, it has been found that an uneven pattern, or a fringe pattern, which will be described in detail later, appeared on the screen when a back surface reflection mirror including a transparent and thin plastic sheet and a reflecting film was used.